P: Unit 2: Autonomic Nervous System

Autonomic Nervous System (ANS)

Overview

The Autonomic Nervous System (ANS) is a critical component of the human nervous system, responsible for regulating involuntary physiological processes including heart rate, blood pressure, respiration, digestion, and sexual arousal. The ANS operates largely below the level of consciousness and is divided into the sympathetic and parasympathetic divisions, each with distinct anatomical and functional characteristics.

Section 1: Structure of the Autonomic Nervous System

1.1 General Anatomy of the Nervous System

• Central Nervous System (CNS): Consists of the brain and spinal cord.

• Peripheral Nervous System (PNS): Comprises all neural structures outside the CNS, including 12 cranial nerves and 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal).

• The PNS is further divided into the somatic (voluntary control of skeletal muscles) and autonomic (involuntary control of smooth muscle, cardiac muscle, and glands) divisions.

Key Terms:

• Neuron: A nerve cell that transmits electrical and chemical signals.

• Ganglion: A cluster of neuron cell bodies in the PNS.

1.2 Reflex Arc / Autonomous Reflex

The reflex arc is the basic functional unit of the nervous system, allowing for rapid, involuntary responses to stimuli.

• Somatic Reflex Arc: Involves a sensory receptor, sensory neuron, integration center (association neuron in CNS), somatic motor neuron (cell body in spinal cord), and effector (skeletal muscle). Neurotransmitter: acetylcholine (ACh), always excitatory.

• Autonomic (Visceral) Reflex Arc: Similar to somatic, but the effector is smooth muscle, cardiac muscle, or glands. Involves two motor neurons: preganglionic (cell body in CNS) and postganglionic (cell body in autonomic ganglion).

Comparison Table: Somatic vs. Autonomic Reflex Arc

Feature	Somatic Reflex	Autonomic Reflex
Number of Motor Neurons	One	Two (pre- and postganglionic)
Effector	Skeletal muscle	Smooth/cardiac muscle, glands
Neurotransmitter	Acetylcholine (ACh)	ACh or Norepinephrine (NE)
Control	Voluntary	Involuntary

1.3 Location of Autonomic Motor Neurons

• Somatic Motor Neurons: Cell bodies in the CNS (spinal cord or brainstem); axons extend directly to skeletal muscles.

• Autonomic Motor Neurons: Two-neuron chain:

  • Preganglionic neuron: Cell body in CNS; axon synapses in autonomic ganglion.

  • Postganglionic neuron: Cell body in ganglion; axon extends to effector organ.

Sympathetic Division

• Preganglionic neurons originate in the thoracic and lumbar regions of the spinal cord (thoracolumbar outflow).

• Most synapse in sympathetic ganglia located near the spinal cord (paravertebral ganglia) or in collateral ganglia.

• Adrenal medulla: Functions as a modified sympathetic ganglion, releasing epinephrine and norepinephrine into the bloodstream.

Parasympathetic Division

• Preganglionic neurons originate in the brainstem (cranial nerves III, VII, IX, X) and sacral spinal cord (craniosacral outflow).

• Ganglia are located near or within the target organs, resulting in long preganglionic and short postganglionic fibers.

Section 2: Functions of the Autonomic Nervous System

2.1 Adrenergic and Cholinergic Synaptic Transmission and Neurotransmitters

• Cholinergic neurons: Release acetylcholine (ACh). Includes all preganglionic neurons (both sympathetic and parasympathetic) and most postganglionic parasympathetic neurons.

• Adrenergic neurons: Release norepinephrine (NE). Most postganglionic sympathetic neurons are adrenergic.

• Catecholamines: Family of molecules including dopamine, norepinephrine, and epinephrine.

2.2 Response to Cholinergic Stimulation

• Nicotinic receptors: Found on postganglionic neurons; always excitatory.

• Muscarinic receptors: Found on effector organs; can be excitatory or inhibitory depending on the organ.

• Cholinergic stimulation generally promotes 'rest and digest' activities (e.g., decreased heart rate, increased digestive activity).

2.3 Response to Adrenergic Stimulation

• Adrenergic receptors: Alpha (α) and beta (β) receptors on effector organs.

• Stimulation of these receptors by NE or epinephrine leads to 'fight or flight' responses (e.g., increased heart rate, dilated pupils, increased blood glucose).

• Some organs have both receptor types, leading to different effects depending on which is activated.

2.4 Organs with Dual Innervation

• Most organs receive input from both sympathetic and parasympathetic divisions, often with opposing effects (antagonistic).

• Examples:

  • Heart: Sympathetic increases rate; parasympathetic decreases rate.

  • Pupil: Sympathetic dilates; parasympathetic constricts.

  • Digestive tract: Sympathetic inhibits; parasympathetic stimulates.

• Some effects are complementary or cooperative (e.g., sexual function: erection by parasympathetic, ejaculation by sympathetic).

2.5 Control of ANS by Higher Brain Centers

• The hypothalamus is the primary control center for the ANS, integrating input from the limbic system and cerebral cortex.

• Other brain regions involved include the brainstem (medulla oblongata, pons) and spinal cord.

• Higher brain centers can influence autonomic responses to emotional states (e.g., stress, fear).

Summary Table: Sympathetic vs. Parasympathetic Divisions

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar (T1-L2)	Craniosacral (brainstem, S2-S4)
Ganglia Location	Near spinal cord	Near/in effector organs
Neurotransmitter (postganglionic)	Norepinephrine (NE)	Acetylcholine (ACh)
General Function	"Fight or flight"	"Rest and digest"
Effect on Heart Rate	Increases	Decreases
Effect on Digestive Activity	Decreases	Increases

Key Concepts and Applications

• Beta-blockers: Drugs that block β-adrenergic receptors, lowering blood pressure but potentially causing bronchoconstriction (contraindicated in asthma).

• Alpha-adrenergic agonists: Used in cold medications to constrict blood vessels and reduce nasal congestion; may raise blood pressure.

• Dual innervation: Allows precise regulation of organ function through antagonistic, complementary, or cooperative effects.

Additional info: The notes above expand on the original content by providing definitions, examples, and tables for clarity and completeness, as would be expected in a mini-textbook study guide for Anatomy & Physiology students.